The demand for gases primarily for energy value and as chemical process raw materials remains strong. Due to tremendous past and ongoing consumption, many of the more easily tapped underground gas formations have matured or even become depleted. Some significant reserves of subterranean gas have been identified but access to them is often restricted by harsh climactic and geographical conditions and production may be further limited by political constraints. Hence, there is a keen commercial interest to recover gas from previously identified, low yielding or partially depleted sources.
Gas in subterranean formations is frequently found mixed with liquid, typically oil and/or water. Under pressure existing in typical gas-yielding formations, the gas is usually dissolved in the liquid and needs to be separated therefrom. Traditional separation processes are usually operated above ground. The energy needed to pump large quantities of gas-bearing water to the surface and to carry out separation can be prohibitively expensive. Furthermore, the disposal of water waste from the separation adds to the cost and can be deleterious to the environment. Cost can be particularly critical in the recovery of gas from low-yielding formations or of the sometimes significant amount of residual gas in previously produced wells. If the cost of obtaining gas becomes too high relative to the value of the gas, producers will abandon a well even though substantial gas remains unrecovered in the formation.
Substantial effort has been dedicated to developing capability to separate gas from liquid in the well and thereby avoid the need to extract, treat and dispose of the liquid. For example U.S. Pat. No. 4,231,767 to Acker discloses a downhole liquid-gas separator having an inverted conical convoluted fine mesh screen in a tubular housing. A liquid-gas mixture enters the housing near the lower apex end of the screen which is of a mesh size selected so that liquid pumps through the screen and gas bubbles channel upward into troughs between screen convolutions. This apparatus is chiefly directed to separating gas from crude oil in oil-producing wells.
U.S. Pat. No. 5,653,286 to McCoy et al. discloses a downhole gas separator aimed at effectively pumping gas-free crude oil from a well. The separator has a tubular body and a decentralizer mounted to one side for driving the opposite side of the body against an interior wall of the well casing. The separator operates on the principle that substantially gas-free liquid will flow in the narrow gap between the opposite side and the casing to be pumped away while gas bubbles flow in the wide gap between the casing wall and the tubular body.
In recent years membrane separation process technology directed toward separating gas mixed with water in wells has developed. U.S. Pat. No. 4,171,017 to Klass teaches a method of separating and recovering methane and other similar fuel components from brine in geopressurized geothermal zones by immersing gas permeable membranes selective for the fuel components in the brine, permitting the fuel components to permeate the membranes while rejecting the brine, and collecting the fuel components.
U.S. Pat. No. 5,673,752 of Scudder et al. discloses an apparatus for separating gas from well fluid solutions which employs a hydrophobic membrane filter element that is permeable to gas but impermeable to water. The apparatus is positioned in a well bore at a depth where hydrostatic pressure allows the gas to bubble out of solution.
Despite great advances in the art represented by the examples cited above, there still remains a need to easily and inexpensively recover substantially water-free gas from subterranean gas producing formations. There is particularly great need for a way to readily harvest residual gas from low-yielding formations, i.e., those having relatively large amounts of water mixed with gas. Accordingly, the present invention provides a gas recovery device comprising a plenum defining a chamber adapted to contain a gas,
at least one permeation unit comprising a gas permeable, liquid impermeable membrane and having an elongated shape defining two ends and an internal cavity adapted to receive gas transported through the membrane and which permeation unit is affixed to the gas recovery device only at one of the two ends which end is operative to transfer gas from the internal cavity to the chamber, and a gas transfer pipe in fluid communication with the chamber and extending away from the plenum. Preferably, the permeation unit has an elongated configuration and is composed of material adapted to permit the permeation unit to flex along its axis of elongation without occluding the internal cavity. Thus, the permeation unit is free to move about and this movement disturbs the permeation boundary conditions so as to enhance the permeation of the gas through the membrane. There is also provided an apparatus comprising a plurality of gas recovery devices strung together.
This invention additionally provides a novel method of separating gas from a fluid of gas and liquid in intimate mutual contact comprising the steps of providing a gas recovery device comprising a plenum defining a chamber adapted to contain a gas, at least one permeation unit comprising a gas permeable, liquid impermeable membrane and having an elongated shape defining two ends and an internal cavity adapted to receive gas transported through the membrane and which permeation unit is affixed to the gas recovery device only at one of the two ends which end is operative to transfer gas from the internal cavity to the chamber, and a gas transfer pipe in fluid communication with the chamber and extending away from the plenum immersing the gas recovery device in the fluid of gas and liquid thereby permitting the fluid to contact one side of the membrane, maintaining contact of the fluid with the membrane to selectively transport gas substantially free of liquid through the membrane into the internal cavity, allowing the gas transported through the membrane to flow into the plenum, and removing the gas through the pipe.